Two stage dry primary pumps are known with each stage generally being constituted by a vane pump. Compared with an oil-lubricated vane pump, a dry vane pump avoids oil vapor migrating into the enclosure to be evacuated, but the main drawback lies in the existence of major dry friction giving rise to rapid wear and accelerated reduction in performance.
Screw type dry primary pumps are also known which are constituted by a single stage. Under steady conditions, such pumps are capable of pumping a fluid from a limiting inlet pressure of about 10.sup.31 2 mbar up to atmospheric pressure. They have the advantage of no contact between the screws and thus no friction, and this makes them very reliable. However, in that portion of the pump where the pressure is greater than 10 mbar they absorb high power and the absorbed power is dissipated mainly as heat. The portions of the screws operating in this high pressure zone (pressures higher than 10 mbar) are therefore subjected to considerable heating which gives rise to major asymmetrical expansion that is incompatible with internal clearance. Increasing the clearance between the two screws is not a satisfactory solution since performance is no longer ensured under such conditions, in particular with respect to pumped throughout and to limiting pressure.
The object of the present invention is to provide a solution to this problem, i.e. to obtain a dry primary pump capable, under steady conditions, of sucking a fluid from a limiting pressure of about 10.sup.31 2 mbar. or even less, and of delivering it at atmospheric pressure, while offering a wide range of pumping rates, from 50 m.sup.3 /h to a few thousand m.sup.3 /h.